Due to the high pressures involved and limited space, master brake cylinders have been conventionally formed as relatively thick-walled but simple castings by utilizing either sand or metal cores. Whichever method or coring is employed, the critical rubbing surfaces of the master brake cylinder must be machined to eliminate scratches resulting from stripping of coring and to define a better surface against which the piston may bear. The cost of machining is one of the undesirable aspects of conventionally made brake cylinders.
To further reduce weight and avoid machining of brake cylinders, an integral composite casting would be desirable provided the high volume material of the composite is an extremely light material. However, with composites it is necessary that the heavier mass material (such as a sleeve defining the inner wall of the pressure cylinder) be bonded extremely tightly to the lighter material or element. Achieving a successful bond is complicated by (a) the metallurgical nature of the materials which may affect the bonding interface, (b) the presence of cast openings that must be defined with precision and which openings extend through both elements of the composites, and (c) a desire for high speed casting which may prohibit use of certain materials and modes of supporting the coring. Several options to the designer of a lighter composite master brake cylinder may include: steel sheet metal, aluminum die casting, plastic, and magnesium. Sheet steel is disadvantageous because of the necessity to additionally protect the exterior against corrosion and the high cost of tooling and capitalization, as well as the need for complicated welding. Aluminum die casting is disadvantageous because a galling problem may occur between the aluminum brake piston and the aluminum cylinder body; the die cast body also requires expensive anodizing. Plastic brake cylinders are disadvantageous because of the flexibility of the material in yielding to the extremely high internal pressures. Magnesium is an excellent candidate because of its low weight and ease of die casting, but presents certain process problems with respect to facilitating a bonded composite design.